Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1 x 10(19) cm(-2)) on the superconducting transition temperature T-c and the upper critical field H-c2 of polycrystalline MgB2 samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T-c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 700 degreesC. A weak change in the dH(c2)/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the "pure" limit of the theory of disordered superconductors. The suppression of T-c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level. (C) 2001 MAIK "Nauka/Interperiodica".